1. Field of the Invention
The invention is related to the general field of electronic fuel control systems for internal combustion engines and, in particular, to that portion of the above field directed to metering fuel to the engine during the transient warm up period.
2. Description of the Prior Art
It is well known in the field of internal combustion engine fuel control systems that engine speed, absolute air intake manifold pressure, engine temperature, as well as other engine parameters, may be used to compute the desired fuel delivery to the engine under both steady state and transient modes of operation. In particular, the prior art has recognized the need for fuel enrichment during the transient warm up period of the engine and makes provisions for enriching the fuel/air mixture during this critical operational phase of the engine. The fuel enrichment compensates for incomplete vaporization of the fuel, for condensation of the delivered fuel on the cold surfaces of the intake manifold, and for the high frictional load of the cold engine. U.S. Pat No. 3,771,502 - "Circuit for Providing Electronic Warm Up Enrichment Fuel Compensation Which is Independent of the Intake Manifold Pressure in an Electronic Fuel Control System", by J. N. Reddy (Nov. 13, 1973) is indicative of the teachings of the prior art. The Reddy patent teaches warm up enrichment as a function of engine temperature independent of the pressure of the air being inhaled by the engine from the intake manifold. The enrichment during warm up may be a simple linear function of the difference between the actual engine temperature and the normal operating temperature or may be a more complex function of engine temperature, as taught by Yoshida et al in U.S. Pat. No. 3,816,717 - "Electrical Fuel Control System for Internal Combustion Engines"(June 11, 1974). The prior art has also recognized the requirement for enriching the fuel/air mixture being delivered to the engine under full load operating conditions. The Reddy patent cited above, as well as many others, teaches full load enrichment as a function of engine speed while the Yoshida et al patent above, and others, teaches full load enrichment as a function of the pressure in the engine's intake manifold.
Although the prior art has independently recognized the requirement of enriching the fuel/air mixture being delivered to the engine during the warm up period as a function of engine temperature and enriching the fuel/air mixture during full load operation as a function of pressure in the engine's intake manifold, the prior art has treated these as independent variables and failed to recognize the requirement to provide increased warm up enrichment as a combined function of both intake manifold pressure and engine temperature. The requirement for an even greater fuel enrichment under high load operating conditions is attributable to several factors. One factor is that the rate of vaporization of fuel is much slower at higher manifold pressures than it is at the lower manifold pressures. This factor further increases the probability of nucleation rather than vaporization of the fuel as the quantity of injected fuel increases. These and other factors, which determine the quantity of fuel required during the warm up period of the engine, are will known to those skilled in the art. The disclosed invention is a warm up circuit providing increased enrichment as a function of both engine temperature and intake manifold pressure.